Leakage control for closed center valves

ABSTRACT

A hydraulic control valve wherein pressure fluid which leaks along the exterior of the movable valve element from a high pressure inlet zone is intercepted before it reaches any service passage of the valve and conducted along a path externally of the valve element to a low pressure return passage in the valve body.

Raud A. Wilke United States Patent s T N m MA 8 mm MA .mfl D E W N U M Ua W M w w m B r m n e V m 137/625 48X 137/625.69X l37/625.69X

[211 App]. No. 873,521

[22] Filed Nov. 3, 1969 [45] Patented Apr. 20, 1971 [73] AssigneeKoehring Company Milwaukee, Wis.

VALVES ABSTRACT: A hydraulic control valve wherein pressure fluid whichleaks along the exterior of the movable valve element 137/625.69 from ahigh pressure inlet zone is intercepted before it reaches Fl6lk 11/07any service passage of the valve and conducted along a path externallyof the valve element to a low pressure return passage in the valve body.

18 Claims, 2 Drawing Figs.

[51] Int..

FIG-.I.

PATENTE D APR 2 0 van SHEET 1 OF 2 NVENTOR 7 A- zZka E Y ATTOR EZYFIG-.2.

PATENTED APR20 I97:

SHEET 2 OF 2 INVENTOQ.

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LEAKAGE COUL FOR CLUSED CENTER VALVES This invention relates tohydraulic control and has more particular reference to control valves ofthe closed center, hold-in-neutral type.

The valve element of a hold-in-neutral control valve prevents operationof a fluid motor governed thereby in the neutral position of the valveelement in its chamber, at which it blocks flow of pressure fluid to andfrom the motor. Because there is no provision for pump output fluidentering the inlet chamber of a closed center control valve such asherein concerned to be returned to a reservoir through an open centerpassage in the valve body, fluid in the inlet chamber of the valveremains at high pressure all during the time the pump is in operationand the valve element is in neutral. Thus pressure ,fluid at highpressure is immediately available for operation of the motor uponshifting of the valve element to one of its operating positions.

One of the problems encountered with closed center valves of this typeresulted from the impossibility of fitting the valve element in itschamber or bore closely enough to prevent fluid at high pressure in theinlet chamber of the valve from leaking along the wall of the bore, pastthe lands on the valve element in the neutral or hold position thereof,to a service port and thence to the governed motor. Such leakage hascaused unwanted operation of the governed motor, and has createdconditions that could become dangerous to life as well as to costlyhydraulically actuated apparatus' With this problem in mind, it is ageneral object of the invention to provide a closed center control valveof the character described wherein the danger of unwanted motoroperation due to leakage thereto of high pressure fluid past the valveelement of the control valve is obviated.

Hence, it is a more specific purpose of this invention to provide aclosed center control valve of the character described wherein theproblem referred to above is solved by intercepting such leakage fluidbefore it reaches a service passage to which the motor is connected, andby conducting such intercepted leakage fluid to a low pressure returnpassage in the body of the valve.

in the most specific sense, it is an object of the invention to providea closed center control valve of the character described wherein suchleakage fluid is exhausted to a low pressure passage along a path lyingwholly externally of the valve element so as to obviate the need forcostly machining thereof, and which path is chiefly comprised of coredpassages in the body of the control valve.

With these observations and objectives in mind, the manner in which theinvention achieves its purpose will be appreciated from the followingdescription and the accompanying drawings, which exemplify theinvention, it being understood that such changes in the specificapparatus disclosed herein may be made as come within the scope of theappended claims.

The accompanying drawing illustrate two complete examples of theembodiments of the invention constructed according to the best modes sofar devised for the practical application of the principles thereof, andin which:

H6. 1 is a sectional view of a closed center control valve embodyingthis invention; and

FIG. 2 is a sectional view of a closed center control valve similar tothat seen in H6. 1, but illustrating a modified embodiment of theinvention.

Referring now to the accompanying drawings, FIG. 1 illustrates a closedcenter hydraulic control valve having a body 5 with a bore 6 thereindefining a valve chamber. An elongated valve element or spool 7 isclosely fitted in the bore for lengthwise sliding movement thereinbetween a neutral or hold position shown, and a pair of operatingpositions at opposite sides of its neutral position.

The control valve chosen for illustration is intended to govern theoperation of a double acting cylinder or other reversible fluid motor(not seen), although it will be appreciated that this invention isequally as well applicable to control valves for single acting service.For double acting service, the body of the valve is provided with a pairof upwardly opening service passages 8 and 9 which are connectable withthe opposite ends of the cylinder, and which communicate with the valvechamber or bore 6 at zones that are spaced apart a substantial distancealong its axis. These passages are selectively communicable with apressure fluid source, such as a pump 10, by means of the valve spool 7,in one or the other of its operating positions.

The output of the pump is fed into an inlet chamber in the body of thevalve, here shown by way of example as comprised of twin branches 11 and12 which open to the bore 6 at locations spaced apart axially thereoffrom one another and intermediate the zones at which the servicepassages communicate with the bore. As is customary in control valvesheretofore available, the valve spool 7 is adapted to communicate theservice passage 8 with the right-hand inlet branch 12 when the valvespool is shifted to its operating position to the right of neutral, andto communicate service passage 9 with the left-hand inlet branch 11 whenthe spool is shifted to its operating position to the left of neutral.In each of these operating positions, the spool is adapted tocommunicate the service passage not connected with the inlet chamberwith an exhaust or return passage 13.

The valve spool 7 has five circumferential grooves therein, at axiallyspaced apart locations, and which, reading from left to right, arenumbered l4, 15, 16, 17 and 18. In the neutral position of the spoolshown, its groove 14 is in register with the junction between the boreand service passage 8, while its groove 17 is in register with thejunction between the bore and service passage 9. The grooves 14 and 17provide for communicating the service passages 8 and 9 with either theinlet chamber or the return passage 13, depending upon the direction thevalve spool is shifted out of its neutral position.

Besides the service passages 8 and 9, the valve body is provided with anumber of other passages through portions of which pressure fluid canflow to and from the service passages in the different operatingpositions of the valve spool. These include the exhaust passage 13mentioned previously, which can be considered as a low pressure bridgeof U-shaped configuration, having a bight portion 20 that extendslengthwise of the bore 6 a distance therebelow, and substantiallyparallel opposite legs which define exhaust branches 21 and 22 thatextend upwardly from the opposite ends of the bight portion andintersect the bore 6 at zones spaced outwardly along the bore from itszones of communication with the service passages 8 and 9, respectively,so as to receive return fluid from said service passages through thegrooves 14 and 17 in the spool.

A bridge passage 24 to supply high pressure fluid to the servicepassages is also formed in the body. The high pressure supply bridge isof inverted U-shaped providing a bight portion 25 which extendslengthwise of the bore 6, a distance thereabove, and opposite legs 26and 27 which extend downwardly from the ends of the bight portion to thebore 6 at locations intermediate the service passages 8 and 9. The leg26 is adjacent to passage 6 and provides for supplying pressure fluidthereto through spool groove 14; while the leg 27 is adjacent to passage9 to provide for supply of pressure fluid thereto through spool groove17. The two branches 11 and 12 of the inlet chamber open to the bore 6at zones between those at which the high pressure bridge legs 26 and 27communicate with the bore.

Substantially medially between the inlet branches 11 and 12, a shortfeeder passage 30 extends upwardly from the bore toward the bightportion 25 of the high pressure bridge 24. This feeder passage iscommunicated with the high pressure bridge through a check valve 31 in apassageway 32 that joins the feeder passage 30 with the left-hand leg 26of the bridge. The check valve 31 is a so-called load holding checkvalve which is arranged to open only at times when pressure of fluid infeeder passage 30 exceeds the pressure of fluid in the high pressurebridge 24. The feeder passage 30 opens to the bore 6 at a zone which isin register with the groove 16 in the valve spool when the spool is inits neutral position.

Feeder passage 1%) is communicated with the inlet branch 12 when thevalve spool is shifted to its operating position to the right of neutralso that pump fluid can then flow to service passage 8 via the left-handleg of the bridge 24. Similarly, passage 30 cooperates with bridge 24 tosupply service passage 9 from inlet branch 11 when the valve spool isshifted to its operating position to the left of neutral.

in the well known open center valves commonly used in the past, one orthe other, or both of the inlet branches l1 and 12 were communicated insome way with an outlet passage 35 in the neutral position of the valvespool. The outlet passage 35 is here shown as located substantiallycentrally of the valve body, between the bore 6 and the bight position20 of the low pressure bridge 13, and it is at all times communicatedwith the latter through a short connecting passageway 35'.

In the control valve here shown, however, the lands 36 and 37 on thespool 7 defined by its grooves l5, l6 and 17, close off the inletbranches l1 and 12 from the bore 6 in the neutral position of the valvespool. Accordingly, as long as the pump 10 remains in operation, itsfull output pressure will obtain in the inlet branches 11 and 12 andsuch fluid can leak along the wall of the bore 6 through the slightclearance space essential to assure proper sliding of the spool therein,toward one or the other of the service passages 8 or 9. If nothing isdone to prevent it, such leakage fluid can seep lengthwise along theexterior of the spool to one of the service passages 8 or 9 and causeunwanted operation of the fluid motor governed by the control valve.Such unwanted operation of the fluid motor can at times be dangerous,even through the motor will then operate at only a slow speed.

According to this invention, any high pressure fluid that leaks alongthe wall of the bore 6 from the inlet branch 11 in a direction towardthe service passage 8 in the neutral position of the valve spool will beintercepted at the junction between the bore and the inner end of aventing passage 40 which has its outer end in communication with theoutlet passage 35. The inner end of the passage 40 opens to the bore 6at a location between itsjunctions with the inlet branch 11 and theleft-hand leg 26 of the high pressure bridge passage 24, and so as to bein register with the groove 15 in the valve spool in the neutralposition of the latter. Hence, any fluid that leaks along the wall ofthe bore from the inlet branch 11 toward the service passage 8 will beintercepted before it reaches said service pasage, and be conducted tothe outlet passage 35 through the venting passage 40.

At this point, it is important to observe that in the neutral positionof the valve spool, its groove 15 is located to communicate theleft-hand branch 26 of the high pressure bridge with the venting passage40. Accordingly, any high pressure fluid in the inlet branch 12 thatleaks along the wall of the bore 6 toward the other service passage 9will be intercepted by and collect in the right-hand leg 27 of the highpressure bridge, from whence it will be free to flow through theremainder of said bridge and spool groove 15 to venting passage 40, andthence to the outlet passage 35.

in like manner, the high pressure bridge is employed to conduct to theventing passage 40 any fluid that leaks out of the inlet branches 11and/or 12 into the feeder passage 30. High pressure leakage fluid infeeder passage 30 will be able to flow through passageway 32 and checkvalve 31 therein to the left-hand leg 26 of the high-pressure bridge,and from there across the spool groove 15 to the outlet passage 35 viapassage 40.

From the above, it will be apparent that the high-pressure bridge,through which pressure fluid must flow to reach the service passages inthe working positions of the valve spool, is vented in the neutralposition of the spool. It thus constitutes a low pressure bridge in theneutral position of the valve spool, and it only becomes a high pressurebridge when the valve spool is moved to one or the other of itsoperating positions. At those times the bridge 24 is cut off from theventing passage 40 either by the spool land 36 or by the land 42 thereondefined by spool grooves 14 and 15.

It is also important to realize that because of the above describedarrangement, a more or less conventional passage in the valve body whichis normally relied upon to conduct high pressure fluid to one of theservice passages in an operating position of the valve spool is herecapable of use in conducting leakage fluid to the reservoir in theneutral position of the valve spool.

An important feature of this invention resides in the fact that thegroove 18 in the valve spool 7 enables the spool to be placed in a floatposition when it is shifted to the left far enough to communicate theservice passages 8 and 9 with one another through the low pressurebridge branches 21 and 22.

The present leakage controlled closed center valve construction enablesthis float position to be achieved in a most facile manner, whereasfloat was either impossible or extremely difficult and costly to obtainin the past with comparable valve constructions. This resulted from thefact that in past closed center valves, the junctions of the servicepassages with the bore were customarily communicated with the returnpassages in the neutral position of the valve spool, thus making itessential to provide complicated and costly pilot operated mechanisms toisolate the ports of the controlled fluid motor from the reservoir.

The closed center control valve illustrated in FIG. 2 has severalfeatures in common with that described above. lt has a bore 6 with avalve spool 70 therein, but the spool has five annular grooves which,reading from left to right, are numbered 45, 46, 47, 48 and 49. Thesegrooves define lands 50 and 51 at opposite sides of groove 45; lands 52and 53 at opposite sides of groove 47; and lands 54 and 55 at oppositesides of groove 49.

The valve has an inlet chamber with but a single feeder branch 56 thatopens to the bore 6 in register with the spool groove 47 in the neutralposition of the spool shown, midway between the legs 26 and 27 of thehigh pressure bridge 24.

In this case, when the valve spool is in neutral, the spool lands 52 and53 block flow of fluid into the bore 6 from the legs 26 and 27 of thehigh pressure bridge; while the lands 51 and 54 extend across thejunctions of the service passages 8 and 9 with the bore to prevent flowof fluid therethrough to and from the service passages.

To achieve the purpose of this invention in the valve of FIG. 2, itsbody is provided with a pair of small U-shaped cores, 76 and 77,associated with the service passages 8 and 9, respectively. The bightportion of the core 76 extends lengthwise of the bore 6, beneath thezone at which the service passage 8 joins with the bore but above thebight portion of the return passage 13. The opposite legs 78 and 79 ofcore 76 extend upwardly from its bight to intersect the bore 6 atopposite sides of the service passage 8. The left-hand leg is positionedto be intennediate the service passage 8 and the adjacent exhaust branch21, in register with spool groove 45', while the right-hand leg 79 ispositioned to be intermediate the service passage 8 and the adjacent leg26 of the high-pressure bridge 24, in register with spool groove 46.

Similarly, the U-shaped core 77 has opposite legs 80 and 81 whichintersect bore 6 and lie at opposite sides of service passage 9, withthe leg 80 intermediate said service passage and the adjacent leg 27 ofthe high pressure bridge, in register with spool groove 80; while theleg 81 is intermediate service passage 9 and the adjacent leg 22 of thelow pressure bridge, in register with spool groove 49. The bight of thecore 77 joins the lower ends its legs 80 and 81, below the bore 6 butabove the bight of the low-pressure bridge 13.

The inlet branch 56 is communicated through a passageway 82 and a loadholding check valve 83 with the bight of the high-pressure bridge 24.

As before, the valve spool 70 is shiftable out of its neutral positionshown at each of a pair of operating positions at opposite sides ofneutral, to effect communication of either service passage 8 or 9 withthe inlet branch 56 via the high pressure bridge, and to effectcommunication of the other service passage with the outlet passage 35via the low-pressure bridge 13.

For example, if the service passage 8 is to be communicated with theinlet or feeder passage 56, and service passage 9 is to be concurrentlyconnected with the return passage 13, the valve spool 70 must be shiftedto its operating position to the right of its neutral position. When soshifted, high-pressure fluid in the inlet or feeder 56 can flow past thecheck valve 83 into the high-pressure bridge 24, through its leg 26 andspool groove as into the leg 79 of cored passage 76, and through the leg7% thereof and spool groove 45 to service passage 8. Pressure fluidreturning to service passage 9 from the controlled motor flows throughspool groove 48 into leg 80 of core 77, and through leg M and spoolgroove 49 to branch 22 of the low pressure bridge 13.

Fluid flow to and from the service passages is reversed when the valvespool is shifted to its operating position to the left of neutral.

The same small U-shaped cores 76 and 77 which provide important segmentsof the paths along which pressure fluid flows both to and from theservice passages 8 and 9 in the operating positions of the valve spool,also comprise the means for conducting to the return passage 13high-pressure fluid that leaks axially outwardly along the wall of thebore 6 from the inlet branch as in the neutral position of the valvespool.

All such leakage fluid flowing along the wall of the bore to one or theother, or both, of the small U-shaped cores 76-77, enters their legs 79or 8th closest to the high pressure bridge legs 26-27, and flows to theupright branches 21-22 of the low-pressure bridge 13 through the remotelegs 78 and 81 of said cores. In other words, the cores 76 and 77conduct the leakage fluid to the low-pressure bridge 113, in bypassrelation to the spool lands 5i and 54 that close off the servicepassages h and 9 from the bore.

From the foregoing description, together with the accompanying drawings,it will be readily apparent to those skilled in the art that thisinvention provides a closed center valve in which lwkage fluid from theinlet chamber of the valve is led to a return or outlet passage throughduct means no part of which is comprised of drilled passages in thevalve spool, and which duct means can comprise parts of the paths alongwhich pressure fluid flows to and from the service passages of thevalve.

Those skilled in the art will appreciate that the invention can beembodied in forms other than as herein disclosed for purposes ofillustration.

The invention is defined by the following claims:

lclaim:

l. A closed center hydraulic control valve having a body with a servicepassage that is selectively communicable with either an inlet or anoutlet passage in one or the other of a pair of operating positions of avalve element in a chamber in which the valve element operates, andwherein said service passage is closed off from both the inlet and theoutlet passage by the valve element in a neutral position thereof,characterized by:

A. said inlet and service passages opening to the valve chamber at zonesspaced from one another;

IB. supply passage means having a portion communicating with saidchamber at a location intermediate said zones, and through whichpressure fluid from the inlet passage must flow to reach the servicepassage in one of said operating positions of the valve element; and

C. means defining a vent passage in the valve body, which opens to saidchamber and which is cooperable with the valve element in the neutralposition thereof to conduct to the outlet passage pressure fluid thatleaks along the wall of said chamber from the inlet passage toward thezone at which the service passage opens thereto.

2. A closed center control valve, characterized by:

A. a body having a valve chamber therein;

l3. service, inlet and outlet passages in the body, opening to saidchamber at spaced apart locations;

C. a valve element in said chamber adapted to close off the inletpassage from the outlet passage in a neutral position of the valveelement at which it is adapted to also close off the service passagefrom both the inlet and outlet passages, said valve element beingadapted to effect communication of the service passage selectively witheither the inlet or the outlet passage in different operating positionsof the valve element;

D. a supply passage which opens to said chamber and is adapted toconduct pressure fluid from the inlet passage to the service passage inone of said operating positions of the valve element; and

E. vent passage means which extends through a portion of the body fromthe outlet passage toward the valve chamber and intersects the latter ata location between its junction with the service and inlet passages,said vent' passage being cooperable with the valve element in theneutral position thereof to conduct to the outlet passage pressure fluidfrom the inlet passage that leaks along the wall of the valve chambertoward its junction with the service passage.

3. A closed center control valve of the type having a body with a valveelement which is operable to effect communication, through a chamber inwhich the valve element operates, of a service passage with either apressure fluid supply passage or with an outlet passage in one or theother of a pair of operating positions to which the valve element ismovable, characterized by:-

A. means on the valve element operable in a hold position thereof toclose off the service passage from both the supply and the outletpassages, and to also close off the supply passage from the outletpassage; and

B. vent passage means in the body comprising a branch of the outletpassage wholly externally of the valve element, said vent passage meansintersecting the valve chamber at a location ahead of its junction withthe service passage, with respect to the direction that fluid from theinlet passage tends to leak toward the service passage along theexterior of the valve element, so as to intercept such leakage fluid andprevent it from reaching the service passage.

4. A closed center hydraulic control valve of the type having a bodywith a service passage that is selectively communicable with either aninlet passage or with an outlet passage in one or the other of a pair ofoperating positions of a valve element in a chamber in which the valveelement operates, and wherein said service passage is closed off fromboth the inlet and the outlet passage by the valve element in a neutralposition thereof, characterized by:

A. said inlet and service passages communicating with the valve chamberat zones spaced from one another;

B. supply passage means having a portion communicating with said chamberat a location intermediate said zones, and through which inlet fluidmust flow to reach the service passage in one of said operatingpositions of the valve element, said supply passage means registeringwith a groove in the exterior of the valve element in the neutralposition thereof; and

C. means in the valve body defining a vent passage opening to saidchamber and registering with said groove in the neutral position of thevalve element, to communicate said supply passage means with the outletpassage along a path wholly externally of the valve element.

5. A control valve of the type having a body with an inlet, an outlet,supply passage means, a movable valve element, and a service passagewhich is selectively communicable with either the inlet or the outlet bythe valve element, characterized by:

A. the supply passage means comprising a supply branch through which theinlet is communicated with the service passage by the valve element inone working position thereof, said supply branch being closed off fromthe inlet and from the service passage in a hold position of the valveelement at which. it closes off the service passage from the outlet;

for the interception of any high pressure fluid that leaks past thevalve element from the inlet or from the supply passage means in saidhold position of the valve element, and being adapted to freely conductall such intercepted leakage fluid to the outlet so as to prevent flowof said said feeder passage being normally closed off from both inletbranches by the valve spool but being communicable with different onesthereof in said operating positions of the spool; and

C. a check valve controlled passage in the body, through which pressurefluid received in the feeder passage from one or the other of said inletbranches can flow to the high pressure supply passage.

10. ln a closed center control valve having a body with a bore and anelongated valve spool slidable endwise in the bore from a neutral closedcenter position to each of a pair of operating positions to selectivelycommunicate either of a pair of service passages with pressure fluidinlet means and the other service passage with outlet meanscharacterized by:

leakage fluid to the service passage.

6. A closed center control valve of the type having a body with inletand outlet passages, a service passage, a valve chamber with which saidpassages communicate at zones spaced from one another, and a movablevalve element in said chamber to control communication of the servicepassage with the inlet and outlet passages, characterized by:

A. said valve element being adapted in a hold position thereof to closeoff communication of the service passage with both the inlet and theoutlet passages, and being adapted to communicate the service passagewith either the inlet or the outlet passage in one or the other of apair of operating positions to which the valve element is movable; and

B. duct means rendered operative by the valve element in said holdposition thereof for conducting to the outlet passage, in bypassrelation to the service passage, pressure fluid from the inlet passagewhich leaks along the exterior of the valve element toward the servicepassage, said duct means being located entirely externally of the valveelement and leading from a zone of said chamber between the zones atwhich said inlet and service passages communicate with said chamber.

7. A closed center valve having a body with a bore and an elongatedvalve spool in the bore movable from a hold position to a pair ofoperating positions to selectively communicate either of a pair ofservice passages with inlet means and the other service passage withreturn means, characterized by:

A. the service passages opening to the bore at first zones spaced apartaxially of the bore;

B. a high-pressure supply passage through which pressure fluid from theinlet means is caused to flow to one of said service passages in each ofsaid operating positions of the valve spool, said supply passage havinga pair of branches, one for each service passage, and which branchesopen to the bore at second zones intermediate but axially adjacent tosaid first zones; and

C. a venting passage in the valve body, wholly externally of 55 A. saidservice passages opening to the bore at first zones spaced apartlengthwise of the bore;

B. said inlet means opening to the bore intermediate said first zones;

C. a supply passage through which the service passages are communicatedwith the inlet means by the valve spool in said operating positionsthereof, said supply passage having a pair of branches, one for eachservice passage, and opening to the bore at second zones each adjacentto but between one of said first zones and the junction of the bore withthe inlet means;

D. said outlet means comprising a return passage having a pair ofbranches, one for each service passage, said branches intersecting thebore at third zones each of which is adjacent to one of said first zonesbut at the side thereof remote from the adjacent second zone, and anoutlet passage which is communicated with both of said return branches;and

E. a venting passage at all times connecting with said outlet means andwith the bore, and cooperating with the valve spool in the neutralposition thereof to vent one of said branches of the supply passage.

11. The closed center control valve of claim 10, wherein said ventingpassage opens to the bore at a location between the inlet means and oneof said second zones and is communicated with said one branch of thesupply passage through a groove in the valve spool and the bore in whichthe spool operates.

12. The closed center control valve of claim 11, further characterizedby:

the valve spool, to nonnally communicate the highpressure supply passagewith the return means, said venting passage opening to the bore at alocation intermediate said second zones but adjacent to one of them, andbeing cooperable with the valve spool in the neutral position thereof tocommunicate one branch of and an elongated valve spool slidable axiallytherein from a closed center neutral position to each of a pair ofoperating positions to communicate either of a pair of service passageswith centrally disposed inlet means and the other service passage withoutlet means through grooves in the valve spool and portions of thebore, characterized by:

the supply passage with the return means through the bore in which thevalve spool operates.

8. The closed center control valve of claim 7, wherein said inlet meansis at all times communicable with the supply passage through a checkvalve arranged to pass pressure fluid into the supply passage; andwherein said inlet means opens to the bore at a location between saidsecond zones.

9. The closed center control valve of claim 7, further characterized by:

A. said inlet means comprising a pair of inlet branches which open tothe bore at axially spaced locations intennediate said second zones;

B. a feeder passage in the body, intersecting said bore at a locationbetween its junctions with said inlet branches, 75

A. the outlet means being centrally located in the body, at

one side of the bore;

B. the service passages extending from the other side of the bore andintersecting the same at first zones which are spaced in axiallyopposite directions from the inlet means;

C. a pair of U-shaped bridge passages in the body, each having oppositebranches that intersect the bore at axially opposite sides of one ofsaid first zones, and a bight at the same side of the bore as the outletmeans;

D. a U-shaped supply passage that embraces the inlet means and intowhich pressure fluid is caused to flow therefrom by the valve spool ineither of its operating positions, said supply passage having oppositebranches that intersect the bore at spacedlocations axially inwardlyfrom the bridge passages, and a bight at the side of the bore remotefrom the outlet means; i

a U-shaped return passage in the body, having opposite branches, one foreachof said bridge passages, said return passage branches intersectingthe bore at zones axially outwardly of the bridge passages and having abight joining its branches and extending substantially lengthwise of thebore at the same side thereof as the outlet means but spaced fartherfrom the bore than the outlet means and the bights of the bridgepassages;

F. a passageway communicating the bight of the return passage with theoutlet means; and

G. in its neutral position, the valve spool having 1. lands that closeoff communication of the service passages and the supply passagebranches with the bore,

2. grooves that communicate theaxially remote branches of said bridgepassages with the branches of the return passage, through the bore inwhich the spool operates,

3. and grooves that register with the innermost branches of the bridgepassages.

14. The closed'center control valve of claim 13, wherein said inletmeans intersects the bore intermediate the branches of the supplypassage.

15. A hydraulic control valve having a' body with a bore, and anelongated valve element shiftable in said bore to diflerent positions toselectively communicate fluid flow passages in the body with oneanother, wherein said fluid flow passages provide:

v A. a pair of service passages that open from one side of the bore atfirst zones spaced apart lengthwise of the bore;

B. a supply passage having a pair of branches, one for each servicepassage, and which branches open to the bore at second zones locatedaxially inwardly of said first zones;

C. a return passage having a pair of branches, one for each servicepassage, and which return branches open to the bore at third zoneslocated axially outwardly of said first zones;

D. inlet passage means providing for flow of supply fluid to the servicepassages via said supply passage under the control of the valve element,said inlet passage means intersecting the bore between the supplypassage lL branches; and

E. vent means intersecting the bore at zones axially inwardly of saidfirst zones and cooperable with the valve element in the neutralposition thereof to conduct leakage fluid from the inlet passage meansto the return passage along a path wholly externally of the valveelement.

16. The hydraulic control valve of claim 15, further characterized by:

A. said service passages opening uninterruptedly to one side of the bodyfrom said bore;

B. means on the valve element rendering said vent means 4 operable onlyin the neutral position of the valve element;

and

C. means on the valve element operable in one position thereof tocommunicate said service passages with one another.

17. The hydraulic control valve of claim [6, wherein the valve elementis operable to effect communication of the service passages with oneanother through the return passage.

18. A closed center control valve of the type having a body with a valveelement which is operable to effect communication, through a chamber inwhich the valve B. vent passa e' means in the body l. coopera le withthe valve element in the neutral position thereof to interceptpressurized supply fluid that leaks along the wall of the chamber beforesuch leakage fluid reaches the junction between the chamber and theservice passage, for conducting such.

intercepted pressure fluid to the outlet passage,

2. and said vent passage being cooperable with the valve element toconduct pressurized supply fluid to the service passage in one of saidoperating positions of the valve element.

1. A closed center hydraulic control valve having a body with a servicepassage that is selectively communicable with either an inlet or anoutlet passage in one or the other of a pair of operating positions of avalve element in a chamber in which the valve element operates, andwherein said service passage is closed off from both the inlet and theoutlet passage by the valve element in a neutral position thereof,characterized by: A. said inlet and service passages opening to thevalve chamber at zones spaced from one another; B. supply passage meanshaving a portion communicating with said chamber at a locationintermediate said zones, and through which pressure fluid from the inletpassage must flow to reach the service passage in one of said operatingpositions of the valve element; and C. means defining a vent passage inthe valve body, which opens to said chamber and which is cooperable withthe valve element in the neutral position thereof to conduct to theoutlet passage pressure fluid that leaks along the wall of said chamberfrom the inlet passage toward the zone at which the service passageopens thereto.
 2. A closed center control valve, characterized by: A. abody having a valve chamber therein; B. service, inlet and outletpassages in the body, opening to said chamber at spaced apart locations;C. a valve element in said chamber adapted to close off the inletpassage from the outlet passage in a neutral position of the valveelement at which it is adapted to also close off the service passagefrom both the inlet and outlet passages, said valve element beingadapted to effect communication of the service passage selectively witheither the inlet or the outlet passage in different operating positionsof the valve element; D. a supply passage which opens to said chamberand is adapted to conduct pressure fluid from the inlet passage to theservice passage in one of said operating positions of the valve element;and E. vent passage means which extends through a portion of the bodyfrom the outlet passage toward the valve chamber and intersects thelatter at a location between its junction with the service and inletpassages, said vent passage being cooperable with the valve element inthe neutral position thereof to conduct to the outlet passage pressurefluid from the inlet passage that leaks along the wall of the valvechamber toward its junction with the service passage.
 2. and said ventpassage being cooperable with the valve element to conduct pressurizedsupply fluid to the service passage in one of said operating positionsof the valve element.
 2. grooves that communicate the axially remotebranches of said bridge passages with the branches of the returnpassage, through the bore in which the spool operates,
 3. and groovesthat register with the innermost branches of the bridge passages.
 3. Aclosed center control valve of the type having a body with a valveelement which is operable to effect communication, through a chamber inwhich the valve element operates, of a service passage with either apressure fluid supply passage or with an outlet passage in one or theother of a pair of operating positions to which the valve element ismovable, characterized by: A. means on the valve element operable in ahold position thereof to close off the service passage from both thesupply and the outlet passages, and to also close off the supply passagefrom the outlet passage; and B. vent passage means in the bodycomprising a branch of the outlet passage wholly externally of the valveelement, said vent passage means intersecting the valve chamber at alocation ahead of its junction with the service passage, with respect tothe direction that fluid from the inlet passage tends to leak toward theservice passage along the exterior of the valve element, so as tointercept such leakage fluid and prevent it from reaching the servicepassage.
 4. A closed center hydraulic controL valve of the type having abody with a service passage that is selectively communicable with eitheran inlet passage or with an outlet passage in one or the other of a pairof operating positions of a valve element in a chamber in which thevalve element operates, and wherein said service passage is closed offfrom both the inlet and the outlet passage by the valve element in aneutral position thereof, characterized by: A. said inlet and servicepassages communicating with the valve chamber at zones spaced from oneanother; B. supply passage means having a portion communicating withsaid chamber at a location intermediate said zones, and through whichinlet fluid must flow to reach the service passage in one of saidoperating positions of the valve element, said supply passage meansregistering with a groove in the exterior of the valve element in theneutral position thereof; and C. means in the valve body defining a ventpassage opening to said chamber and registering with said groove in theneutral position of the valve element, to communicate said supplypassage means with the outlet passage along a path wholly externally ofthe valve element.
 5. A control valve of the type having a body with aninlet, an outlet, supply passage means, a movable valve element, and aservice passage which is selectively communicable with either the inletor the outlet by the valve element, characterized by: A. the supplypassage means comprising a supply branch through which the inlet iscommunicated with the service passage by the valve element in oneworking position thereof, said supply branch being closed off from theinlet and from the service passage in a hold position of the valveelement at which it closes off the service passage from the outlet; B.return passage means through which the service passage is communicablewith the outlet in a second working position of the valve element butwhich is closed off from the service passage in said hold position ofthe valve element; and C. a passageway in the body through which saidsupply passage means is communicated with the outlet by the valveelement, externally of the latter, in said hold position of the valveelement, said passageway providing for the interception of any highpressure fluid that leaks past the valve element from the inlet or fromthe supply passage means in said hold position of the valve element, andbeing adapted to freely conduct all such intercepted leakage fluid tothe outlet so as to prevent flow of said leakage fluid to the servicepassage.
 6. A closed center control valve of the type having a body withinlet and outlet passages, a service passage, a valve chamber with whichsaid passages communicate at zones spaced from one another, and amovable valve element in said chamber to control communication of theservice passage with the inlet and outlet passages, characterized by: A.said valve element being adapted in a hold position thereof to close offcommunication of the service passage with both the inlet and the outletpassages, and being adapted to communicate the service passage witheither the inlet or the outlet passage in one or the other of a pair ofoperating positions to which the valve element is movable; and B. ductmeans rendered operative by the valve element in said hold positionthereof for conducting to the outlet passage, in bypass relation to theservice passage, pressure fluid from the inlet passage which leaks alongthe exterior of the valve element toward the service passage, said ductmeans being located entirely externally of the valve element and leadingfrom a zone of said chamber between the zones at which said inlet andservice passages communicate with said chamber.
 7. A closed center valvehaving a body with a bore and an elongated valve spool in the boremovable from a hold position to a pair of operating positions toselectively communicate either of a pair of service passages with inletmeans and the oTher service passage with return means, characterized by:A. the service passages opening to the bore at first zones spaced apartaxially of the bore; B. a high-pressure supply passage through whichpressure fluid from the inlet means is caused to flow to one of saidservice passages in each of said operating positions of the valve spool,said supply passage having a pair of branches, one for each servicepassage, and which branches open to the bore at second zonesintermediate but axially adjacent to said first zones; and C. a ventingpassage in the valve body, wholly externally of the valve spool, tonormally communicate the high-pressure supply passage with the returnmeans, said venting passage opening to the bore at a locationintermediate said second zones but adjacent to one of them, and beingcooperable with the valve spool in the neutral position thereof tocommunicate one branch of the supply passage with the return meansthrough the bore in which the valve spool operates.
 8. The closed centercontrol valve of claim 7, wherein said inlet means is at all timescommunicable with the supply passage through a check valve arranged topass pressure fluid into the supply passage; and wherein said inletmeans opens to the bore at a location between said second zones.
 9. Theclosed center control valve of claim 7, further characterized by: A.said inlet means comprising a pair of inlet branches which open to thebore at axially spaced locations intermediate said second zones; B. afeeder passage in the body, intersecting said bore at a location betweenits junctions with said inlet branches, said feeder passage beingnormally closed off from both inlet branches by the valve spool butbeing communicable with different ones thereof in said operatingpositions of the spool; and C. a check valve controlled passage in thebody, through which pressure fluid received in the feeder passage fromone or the other of said inlet branches can flow to the high pressuresupply passage.
 10. In a closed center control valve having a body witha bore and an elongated valve spool slidable endwise in the bore from aneutral closed center position to each of a pair of operating positionsto selectively communicate either of a pair of service passages withpressure fluid inlet means and the other service passage with outletmeans characterized by: A. said service passages opening to the bore atfirst zones spaced apart lengthwise of the bore; B. said inlet meansopening to the bore intermediate said first zones; C. a supply passagethrough which the service passages are communicated with the inlet meansby the valve spool in said operating positions thereof, said supplypassage having a pair of branches, one for each service passage, andopening to the bore at second zones each adjacent to but between one ofsaid first zones and the junction of the bore with the inlet means; D.said outlet means comprising a return passage having a pair of branches,one for each service passage, said branches intersecting the bore atthird zones each of which is adjacent to one of said first zones but atthe side thereof remote from the adjacent second zone, and an outletpassage which is communicated with both of said return branches; and E.a venting passage at all times connecting with said outlet means andwith the bore, and cooperating with the valve spool in the neutralposition thereof to vent one of said branches of the supply passage. 11.The closed center control valve of claim 10, wherein said ventingpassage opens to the bore at a location between the inlet means and oneof said second zones and is communicated with said one branch of thesupply passage through a groove in the valve spool and the bore in whichthe spool operates.
 12. The closed center control valve of claim 11,further characterized by: A. the branches of said supply passage beingconnected with one another at one side of The bore; B. the branches ofsaid return passage being connected by a return passage portion whichextends generally lengthwise of the bore at its opposite side; and C.said outlet passage being located between the bore and said passageportion that connects the branches of the return passage.
 13. A closedcenter control valve having a body with a bore and an elongated valvespool slidable axially therein from a closed center neutral position toeach of a pair of operating positions to communicate either of a pair ofservice passages with centrally disposed inlet means and the otherservice passage with outlet means through grooves in the valve spool andportions of the bore, characterized by: A. the outlet means beingcentrally located in the body, at one side of the bore; B. the servicepassages extending from the other side of the bore and intersecting thesame at first zones which are spaced in axially opposite directions fromthe inlet means; C. a pair of U-shaped bridge passages in the body, eachhaving opposite branches that intersect the bore at axially oppositesides of one of said first zones, and a bight at the same side of thebore as the outlet means; D. a U-shaped supply passage that embraces theinlet means and into which pressure fluid is caused to flow therefrom bythe valve spool in either of its operating positions, said supplypassage having opposite branches that intersect the bore at spacedlocations axially inwardly from the bridge passages, and a bight at theside of the bore remote from the outlet means; E. a U-shaped returnpassage in the body, having opposite branches, one for each of saidbridge passages, said return passage branches intersecting the bore atzones axially outwardly of the bridge passages and having a bightjoining its branches and extending substantially lengthwise of the boreat the same side thereof as the outlet means but spaced farther from thebore than the outlet means and the bights of the bridge passages; F. apassageway communicating the bight of the return passage with the outletmeans; and G. in its neutral position, the valve spool having
 14. Theclosed center control valve of claim 13, wherein said inlet meansintersects the bore intermediate the branches of the supply passage. 15.A hydraulic control valve having a body with a bore, and an elongatedvalve element shiftable in said bore to different positions toselectively communicate fluid flow passages in the body with oneanother, wherein said fluid flow passages provide: A. a pair of servicepassages that open from one side of the bore at first zones spaced apartlengthwise of the bore; B. a supply passage having a pair of branches,one for each service passage, and which branches open to the bore atsecond zones located axially inwardly of said first zones; C. a returnpassage having a pair of branches, one for each service passage, andwhich return branches open to the bore at third zones located axiallyoutwardly of said first zones; D. inlet passage means providing for flowof supply fluid to the service passages via said supply passage underthe control of the valve element, said inlet passage means intersectingthe bore between the supply passage branches; and E. vent meansintersecting the bore at zones axially inwardly of said first zones andcooperable with the valve element in the neutral position thereof toconduct leakage fluid from the inlet passage means to the return passagealong a path wholly externally of the valve element.
 16. The hydrauliccontrol valve of claim 15, further characterized by: A. said servicepassages opening uninterruptedly to one side of the body from said bore;B. means on the valve element rendering said vent means operable only inthe neutral position of the valve element; and C. means on the valveelement operable in one position thereof to communicate said servicepassages with one another.
 17. The hydraulic control valve of claim 16,wherein the valve element is operable to effect communication of theservice passages with one another through the return passage.
 18. Aclosed center control valve of the type having a body with a valveelement which is operable to effect communication, through a chamber inwhich the valve element operates, of a service passage with either apressure fluid supply passage or with an outlet passage in one or theother of a pair of operating positions to which the valve element ismovable, characterized by: A. means on the valve element operable in ahold position thereof to close off the service passage from both thesupply and the outlet passages, and to also close off the supply passagefrom the outlet passage; and B. vent passage means in the body